Coherent nanointerfaces in thermoelectric materials

Advanced Materials, 2006

Thermoelectric (TE) power generation has come to be appreciated as an attractive means of low-cost conversion of waste heat to useful electrical energy with a small environmental impact. For a compound to qualify as an efficient thermoelectric material it should exhibit the highest TE figure of merit, ZT, possible at the temperature of operation, T. ZT is defined as

J. Mater. Chem. A, 2014

SnTe–AgBiTe2 is not only a solid solution but a nanocomposite. The alloying effect coupled with intense interface scattering leads to considerably decreased lattice thermal conductivity. Bi is much more powerful in neutralizing holes than Sb, giving rise to a much higher Seebeck coefficient. A high ZT was then obtained.

Crystals, 2017

We investigate the microstructure evolution of Ag-alloyed PbTe compounds for thermoelectric (TE) applications with or without additions of 0.04 at. % Bi. We control the nucleation and temporal evolution of Ag 2 Te-precipitates in the PbTe-matrix applying designated aging heat treatments, aiming to achieve homogeneous dispersion of precipitates with high number density values, hypothesizing that they act as phonon scattering centers, thereby reducing lattice thermal conductivity. We measure the temperature dependence of the Seebeck coefficient and electrical and thermal conductivities, and correlate them with the microstructure. It is found that lattice thermal conductivity of PbTe-based compounds is reduced by controlled nucleation of Ag 2 Te-precipitates, exhibiting a number density value as high as 2.7 × 10 20 m −3 upon 6 h aging at 380 • C. This yields a TE figure of merit value of ca. 1.4 at 450 • C, which is one on the largest values reported for n-type PbTe compounds. Subsequent aging leads to precipitate coarsening and deterioration of TE performance. Interestingly, we find that Bi-alloying improves the alloys' thermal stability by suppressing microstructure evolution, besides the role of Bi-atoms as electron donors, thereby maintaining high TE performance that is stable at elevated service temperatures. The latter has prime technological significance for TE energy conversion.

Chemistry of Materials, 2008

We have performed a comparative investigation of the Ag 1-x Pb 18 MTe 20 (M) Bi, Sb) (x) 0, 0.14, 0.3) system to assess the roles of Sb and Bi on the thermoelectric properties. Detailed charge transport data including electrical conductivity, the Seebeck coefficient, the Hall coefficient, and thermal conductivity are presented. Optical reflectivity data support the conclusions of the transport studies. For comparable nominal compositions, the carrier concentrations are lower in the Sb analogs and the mobilities are higher. The Seebeck coefficient decreases dramatically in going from Sb to Bi. High resolution transmission electron microscopy (TEM) images of both samples reveal that all systems contain compositional fluctuations at the nanoscopic level and are nanostructured. Compared to PbTe, the lattice thermal conductivity of AgPb 18 BiTe 20 is substantially reduced. The lattice thermal conductivity of the Bi analog is, however, higher than the Sb analog, and this correlates with the decrease in the degree of mass fluctuation between the nanostructures and the matrix (for the Bi analog). As a result the dimensionless figure of merit ZT of Ag 1-x Pb 18 BiTe 20 is found to be substantially smaller than that of Ag 1-x Pb 18 SbTe 20. Electronic structure calculations performed within the density functional theory and generalized gradient approximation show marked differences in the band structure near the Fermi level between the two analogs providing useful insights on the carrier transport in these systems.

Physical Review B, 2010

Complex multicomponent systems based on PbTe, SnTe, and GeTe are of great interest for infrared devices and high-temperature thermoelectric applications. A deeper understanding of the atomic and electronic structure of these materials is crucial for explaining, predicting, and optimizing their properties, and to suggest new materials for better performance. In this work, we present our first-principles studies of the energy bands associated with various monovalent (Na, K, and Ag) and trivalent (Sb and Bi) impurities and impurity clusters in PbTe, SnTe, and GeTe using supercell models. We find that monovalent and trivalent impurity atoms tend to come close to one another and form impurity-rich clusters, and the electronic structure of the host materials is strongly perturbed by the impurities. There are impurity-induced bands associated with the trivalent impurities that split off from the conduction-band bottom with large shifts towards the valence-band top. This is due to the interaction between the p states of the trivalent impurity cation and the divalent anion which tends to drive the systems towards metallicity. The introduction of monovalent impurities (in the presence of trivalent impurities) significantly reduces (in PbTe and GeTe) or slightly enhances (in SnTe) the effect of the trivalent impurities. One, therefore, can tailor the band gap and band structure near the band gap (hence transport properties) by choosing the type of impurity and its concentration or tuning the monovalent/trivalent ratio. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the whole class of PbTe-, SnTe-, and GeTe-based bulk thermoelectrics.

Journal of Applied Physics, 2010

The concept of energy filtering of the carriers to control the thermoelectric properties of PbTe is experimentally applied in this present work. The energy barriers at the grain interfaces of the nanocomposites and the embedded Ag-rich nanodots within the grains are supposed to control the energy dependency of carrier scattering: that is what we mean by energy filtering of carriers. As a case study, vertical Bridgman grown bulk PbTe:undoped, PbTe:Ag crystals and nanocomposites of PbTe:Ag are used as samples.

arXiv (Cornell University), 2023

The present study demonstrates a large enhancement in the Seebeck coefficient and ultralow thermal conductivity (TE) in Sb 2 Te 3-AgSbTe 2 nanocomposite thin film. The addition of Ag leads to the in-situ formation of AgSbTe 2 secondary phase nano-aggregates in the Sb 2 Te 3 matrix during the growth resulting in a large Seebeck coefficient and reduction of the thermal conductivity. A series of samples with different amounts of minor AgSbTe 2 phases are prepared to optimize the TE performance of Sb 2 Te 3 thin films. Based on the experimental and theoretical evidence, it is concluded that a small concentration of Ag promotes the band flattening and induces a sharp resonatelike state deep inside the valence band of Sb 2 Te 3 , concurrently modifying the density of states (DOS) of the composite sample. In addition, the electrical potential barrier introduced by the band offset between the host TE matrix and the secondary phases promotes strong energy-dependent carrier scattering in the composite sample, which is also responsible for enhanced TE performance. A contemporary approach based on scanning thermal microscopy is performed to experimentally obtain thermal conductivity values of both the in-plane and cross-plane directions, showing a reduced in-plane thermal conductivity value by ~ 58% upon incorporating the AgSbTe 2 phase in the Sb 2 Te 3 matrix. Benefitting from the synergistic manipulation of electrical and thermal transport, a large ZT value of 2.2 is achieved at 375 K. The present study indicates the importance of a combined effect of band structure modification and energy-dependent charge carrier scattering along with reduced thermal conductivity for enhancing TE properties.

Advanced Functional Materials, 2009

Recently, nanoscale inclusions were discovered in alloys of the form AgPb m SbTe 2þm , or (AgSbTe 2 )(PbTe) m , (LAST-m ¼ lead-antimony-silver-tellurium) that were proposed to correlate

ACS Applied Materials & Interfaces, 2018

Capturing and converting waste heat into electrical power through thermoelectric generators based on the Seebeck effect is a promising alternative energy source. Among thermoelectric compounds, PbTe can be alloyed and form precipitates by aging at elevated temperatures, thus reducing thermal conductivity by phonon scattering. Here, PbTe is alloyed with Ag to form Ag-rich precipitates having a number density controlled by heat treatments. We employ complementary scanning transmission electron microscopy and

Acta Materialia, 2012

Pb-alloyed AgSbTe 2 (Pb x Ag 20 Sb 30Àx Te 50 (x = 3, 4, 5 and 6)) composites were synthesized using a modified Bridgman method with a graphite mold to form plate-like samples. The Bridgman-grown specimens were dense, with few solidification cavities, and were sufficiently mechanically robust for a variety of electronic/thermal transport measurements. Inhomogeneity was found on the grain boundary, and was embedded with the nanoprecipitates of d-Sb 2 Te with a feature size of 100 nm of the 5 at.% Pb and 6 at.% Pb specimens. A combined effect of alloying, inhomogeneity and nanoprecipitates leads to a low thermal conductivity of 0.3-0.4 W m À1 K À1 , which approaches the theoretical minimum thermal conductivity of the amorphous material (j min $ 0.36 W m À1 K À1 ). A peak of the zT value, ranging from 0.7 to 0.8, is achieved at 425 K. Further annealing at 673 K increases the grain size and causes a reduction in the value of the zT peak to 0.4.